drm/msm: debugfs interface for sde connector to transfer cmds

This change implements a new feature to transfer commands
to the panel from sde connector level interface functions.
Sde connector initialization module opens debugfs interface
for all the connectors those have support for cmd_transfer
operation.

For example, Sde connector init module creates tx_cmd debugfs file at
/<debugfs-root>/dri/0/DSI-1/ for DSI-1 connector.

Format for DSI command transfer:
echo "series of command bytes" > /<debugfs-root/dri/0/DSI-1/tx_cmd
byte-0: data-type
byte-1: last command. always 0x01
byte-2: channel number
byte-3: flags. MIPI_DSI_MSG_*
byte-4: 0x00
byte-5 and byte-6: command payload length
byte-7 to byte-[7+payload length]: command payload

Example: dcs color invert command transfer to dsi panel
echo "0x05 0x01 0x00 0x02 0x00 0x00 0x02 0x21 0x00" > tx_cmd

The command transfer operations are allowed only if controller
(ex. DSI controller) is in active state.

Read command transfer status:
cat /<debugfs-root>/dsi/0/tx_cmd
returns the status of the last tx_cmd write operation.
0 - failure 1 - success.

The current implementation supports only DSI connectors.

Change-Id: Icbdcf6dd0400f1664abfabcf8f98216d69f56a34
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>